1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a semiconductor thin film, and more particularly to a method of manufacturing a thin film transistor (TFT) using a crystalline silicon film.
In the present specification, the semiconductor device is directed to all of devices that function using semiconductor, and includes not only a single device such as a TFT but also an electro-optic device, an electronic device on which the electro-optic device is mounted, etc.
2. Description of the Related Art
In recent years, thin film transistors used in an electro-optic device such as an active matrix liquid crystal display device have been actively developed.
The active matrix liquid crystal display device is directed to a monolithic display device having a pixel matrix circuit and a driver circuit on the same substrate. Also, a system on-panel in which a logic circuit such as a memory circuit or a clock generating circuit is built has been progressively developed.
Since the driver circuit and the logic circuit of the above type require high-speed operation, it is improper to use an amorphous silicon film as an active layer. For that reason, in the existing circumstances, a TFT using a crystalline silicon film (polysilicon film) as an active layer is being mainly used.
The present inventors disclose a technique in Japanese Patent Unexamined Publication No. Hei 8-78329 as a technique for obtaining a crystalline silicon film on a glass substrate. The technique disclosed in that publication is that catalytic elements that promote crystallization are selectively added to an amorphous silicon film, and a heat treatment is then conducted on the film to form a crystalline silicon film that spreads from a catalytic element added region as a starting point.
This technique makes it possible to lower the crystallization temperature of the amorphous silicon film down to 50 to 100.degree. C. due to the action of the catalytic elements, and also makes it possible to reduce a period of time required for crystallization down to 1/5 to 1/10. Further, because the crystallization of a silicon film progresses laterally substantially in parallel with a substrate surface, the present inventors call this crystallization region "lateral growth region".
Since the catalytic elements are not directly added to the lateral growth region, the catalytic elements remaining in the film is less than that in the case where the catalytic elements are directly added. For example, in the case of directly adding the catalytic elements, the catalytic elements are contained in the film in the orders of 10.sup.19, but in case of the lateral growth region, the orders in which the catalytic elements are contained are 10.sup.18 which is reduced one digit.
The above crystallizing technique enables a silicon film having excellent crystallinity to be obtained at a relatively low temperature. However, since the catalytic elements are contained in the film, to control the amount of introducing the catalytic elements is delicate, thereby leading to a problem on reproducibility and stability (stability of the electric characteristics of an obtained device).
Also, an influence of the catalytic elements remaining in the film cause such problems that the characteristics of the obtained semiconductor device are varied as a time elapses, that an off-state value which is a characteristic value of the thin-film transistor is large.
As the catalytic elements used for promoting the crystallization, there are used metal elements such as nickel or cobalt, and if the catalytic elements are even slightly contained in the film, they adversely affect the electric characteristic and the reliability of the TFT.
Accordingly, it is ideally desirable that the catalytic elements are perfectly removed from the film after a crystallizing process.
Under the above circumstances, in order to reduce the concentration of catalytic elements existing in the crystallized crystalline silicon film, elements selected from Group XV (15) are selectively added to the crystalline silicon film, and a heat treatment is conducted on that film under an oxygen atmosphere, to thereby make the concentration of catalytic elements reduce. However, the catalytic elements cannot be completely removed from the interior or the surface of the film by only the above process, and also in a process of conducting a heat treatment on the crystalline silicon film in an oxygen atmosphere, the catalytic elements existing in the film react with oxygen to form an oxide, and there appears a phenomenon that the oxide abnormally grows on the surface and in the interior of the film.
The oxide that has abnormally grown has a size of several .mu.m, and in the case of using a mask made of the oxide, because the oxide is removed together with the mask in a mask removing process for forming a semiconductor island region, holes are produced in the surface and interior of the film, thereby leading to the failure of the characteristic of the thin film transistor.
As described above, the oxide that abnormally grows causes a problem in forming a semiconductor island region.